Conventionally, a bumper for a motor vehicle is provided with a bumper beam as a structural element which ensures the necessary mechanical strength and rigidity of the bumper. Typically, the bumper beam is covered by a bumper exterior which conforms with the exterior finish of the vehicle body. As a part of an effort to reduce the weight of the vehicle, it has been proposed to use high strength aluminum alloys for bumper beams. Such a bumper beam can be fabricated by extruding a 6000 or 7000 series aluminum alloy into a channel member having a box-shaped cross section, and bending and machining the member into a desired shape. Thereafter, it is necessary to anneal it or otherwise thermally process it so that the bumper beam may have desired mechanical properties.
However, an extruded bumper beam member cannot be easily bent into a desired shape because it has a box-shaped cross section having a large moment of inertia, and is highly resistant bending deformation. In addition to this difficulty, fabricating a bumper beam out of an aluminum alloy involves other difficulties. An extruded aluminum member having a relatively large cross sectional area and a box-shaped cross section cannot be freely bent into a desired shape which the vehicle exterior design dictates without any ordinary means, and a desired dimensional precision cannot be easily achieved. Also, such an extruded aluminum member is not suitable for heat treatment.
Further, the use of a relatively high-strength aluminum significantly reduces the service life of the extrusion die, and this impairs the production efficiency of the bumper. Further, different extrusion dies are needed for different vehicle models to which the bumper are to be used, and the need for stocking a large number of extrusion dies also impairs the production efficiency of the bumper.